The present invention relates generally to the preparation of compositions and systems for the topical delivery of active substances to the skin. More particularly, the invention relates to the preparation of a rigid polymer bead delivery system for active substances such as ultraviolet absorbants, insect repellants, steroids, acne treatments, epidermal lipid replacements, counterirritants, hair growth promotents, and the like.
It is frequently desirable to topically deliver active ingredients to the human skin. In many cases, the active ingredients can be applied directly to the skin, either in a substantially pure form or in combination with a liquid vehicle. Such direct application, however, is limited in a number of respects. First, direct application allows rapid evaporation of volatile active substances, such as those listed above. Second, application of the active substances in substantially pure form can often cause toxic and/or allergic reactions, particularly in the case of infrared absorbants, insect repellants and steroids. While such adverse reactions can often be minimized by dilution of the active substance in a suitable liquid carrier, the consequent decrease in concentration frequently limits the effectiveness of the resulting combination for the intended purpose. Finally, many topically applied active substances have undesirable characteristics, such as an oily feel or a strong odor. While dilution of the pure active substance in a suitable liquid carrier can minimize such aesthetic objections, the resulting dilution will also reduce the effectiveness of the final product.
For these reasons, it would be desirable to provide delivery compositions or systems capable of providing controlled and prolonged delivery of active substances after they have been applied to the skin. Desirably, such delivery systems should also control any odor or toxicity which may be associated with the active substance, and should be suitable both for direct application to the skin and for combination in conventional liquid carriers.
Polymeric beads have been proposed for incorporating various active substances. European Patent No. 61,701 describes the preparation of relatively non-rigid polymeric beads for incorporating active substances, exemplified by emollients. Although such polymer delivery system will likely result in prolonged release of an active substance, the non-rigid beads allow the internal pore network incorporating the active substance to collapse as the substance is released, usually resulting in the entrapment and waste of residual active substance. Also, the European patent teaches a preparation procedure which requires the presence of the active substance during the polymerization of the bead material. Such a preparation procedure is unsuitable for heat and/or radiation labile active substances which will be inactivated under the polymerization conditions.
It would therefore be desirable to provide for polymeric bead delivery systems comprising relatively rigid polymeric beads which allow for substantially complete release of the active ingredient from the pore network of the beads. It would be particularly desirable if such bead delivery systems could be prepared prior to the introduction of the active substance so that the active substance is not exposed to relatively harsh polymerization conditions.